1. Field of the Invention
The present invention relates to a display apparatus with a built-in driver, in which a matrix of TFTs (thin film transistors) using a polycrystalline semiconductor layer are arranged on a display section and a driver formed by similar TFTs is also arranged on the periphery of the apparatus.
2. Description of the Related Art
Recently developed LCDs with a built-in driver include a matrix display section and peripheral drivers formed on the same substrate using a channel layer made of polysilicon (p-Si) in TFTs. The polysilicon film is fabricated by heating, fusing, and re-crystallizing a deposited amorphous silicon film by excimer laser annealing. The TFTs made of such polysilicon films are desirable elements for forming a driver, since such TFTs have a superior driving ability compared to other TFTs made of amorphous silicon.
FIG. 5 is a plan view showing an outlook of such an LCD panel. A display section 2 provided in the center of a substrate 1 includes gate lines (GL) and drain lines (DL) arranged in a matrix, and display pixels 3 are disposed at respective intersections of these lines, each display pixel consisting of a TFT, a pixel capacitor, and the like. A gate driver 4 and a drain driver 5 are disposed on the periphery of the display section 2 along the edge of the substrate 1, and electrode lines 7 extend to respective driver circuits for transmitting various signals from connecting terminals 6 arranged on one side of the substrate 1. A reference number 8 indicates a precharge driver for the drain lines (DL).
The above-mentioned substrate 1 consists of two substrates facing each other. Display pixels 3, the gate driver 4, the drain driver 5, and the like are arranged on one substrate on the side facing the other substrate. A color filter, a light shielding film, and the like are disposed on the other substrate on the side facing the one substrate. The two substrates are bonded together so as to leave a certain gap between the substrates by applying a sealing material 9 on the periphery of the substrate 1, whereby liquid crystal is sealed and kept in the gap. A portion of the sealing material 9 is left open to form an opening section 10 through which liquid crystal is injected to the interior of the substrate 1, as indicated by an arrow 11 in FIG. 5. After injection, a sealing material 12 is applied to seal the opening section 10.
The gate driver 4 and the drain driver 5 are formed by CMOS circuits made of TFTs, and formed integrally on the same substrate using p-si, as in the TFTs of the pixel section.
In such a case, however, upon injection of the liquid crystal into the substrate 1, changes of a threshold value Vt is large for the TFTs located in the vicinity of the injection hole 10 (indicated by a number 13 in FIG. 5) compared to other TFTs. This may cause a circuit operation failure and decrease a manufacturing yield of the display apparatus, if such TFTs are used in a high operating speed circuit, such as the drain driver 5.
The reason for this may be the accumulation of charge on the surface of an isolating film of the TFT induced by a very little amount of water or impurities contained in the liquid crystal material. Specifically, in the vicinity of the injection hole 10, the liquid crystal continues flowing after injection until it fills the substrate 1. As the amount of liquid crystal flowing over such TFTs increases, the amount of charge to be accumulated increases proportionally to form a back channel of the TFTs and largely change the threshold value Vt.